Diode laser bars (or edge-emitting diode lasers) are produced on the basis of 1″ to 4″ (1 to 4 inches=2.45 cm to 10.16 cm) wafers or fragments thereof. FIG. 1 shows a wafer 1 which has already been processed and from which the diode laser bars are produced.
The light from the diode lasers is emitted in the wafer plane. Therefore, the wafers 1 are broken down by a cleaving process. Thus, the wafer 1 can first be broken down into fields 2 (FIG. 2) which are then each broken down into blocks 3 (FIG. 3). It is known that the wafer 1 can also be broken down immediately into the blocks 3 without taking the detour via the fields 2.
The blocks 3 thus produced (which are also named bars) contain the individual diode lasers or the individual emitters. The cleavage surfaces are the emitting surfaces (what are known as light-issuing windows or facets). In order to protect these emitting surfaces and for targeted alteration of the reflective capacity of the two cleavage surfaces, these surfaces have to be coated with, for example, dielectric layers.
For the necessary coating process, the individual blocks 3 (bars) are stacked one above another. In FIG. 4 the stacked bars 3 are shown schematically in a holding device 4. The front (FIG. 5a) and the back (FIG. 5b) of the bars 3 stacked in the holding device 4 are then coated. The cleavage surfaces to be coated of the blocks 3 form, after stacking over all the stacked blocks 3, an almost closed surface. The distance between two blocks lying one above the other is so small that a film which is closed over all the stacked blocks is produced during a coating process. The direct stacking of the blocks one atop another leads to contact between metal layers located on the upper side and underside of the blocks. The oscillations in the coating installations then cause the blocks to stick to one another (high adhesion as a result of micro-cold welding). These two processes, the formation of a closed coating film over all the blocks of a stack and the sticking of the blocks to one another, lead to the following drawbacks:
The sticking of the blocks to one another greatly impedes or prevents unstacking after the coating. FIG. 6 shows the unstacked blocks 3 schematically. The blocks can no longer be separated or can be separated only by appropriate mechanical aids and levers. Both lead to marked contaminations of the blocks and their coated facets and thus to marked losses.
At the same time, the closed layer of the coating leads to grave difficulties during the separation of the individual blocks stacked one above another. It is possible to separate the blocks only by tearing the film. This tearing is an uncontrolled process which can greatly reduce the ability of the coating to adhere to the cleavage surfaces and/or leads to damage to the coating of the facet. Both lead to a reduction in yield and curtail the service life of the diode lasers.
Furthermore, an offset and/or twisting can occur between the blocks during stacking of the blocks 3. This offset and/or twisting leads to shading of parts of the blocks during the coating. Blocks 3 affected thereby must be discarded as defective products.
The formation of a closed film on the facets and the sticking of the blocks on one another are currently prevented as a result of the fact that a shorter dummy block or simply spacer pieces are placed between two blocks. This produces a gap between the blocks and it is not possible for a closed surface to be formed from the coating material. This eliminates the difficulty of the tearing of the coating surface.
The blocks are also prevented from sticking to one another as a result of the fact that a different, generally very smooth, material is positioned between the metal layers of the blocks.
The drawback of this method is:
a) A reduction in the capacity of the coating process by 50% (half of the coated blocks are dummy blocks). At on average 40 hours for a complete coating process (stacking of the blocks, coating of the fracture surfaces and unstacking of the blocks), that is a major cost factor in production.b) In order to coat the same number of blocks, twice the time is required for stacking and unstacking the blocks.c) The dummy blocks produce additional costs (material, procurement, machining, disposal).d) The dummy blocks are generally used a plurality of times in order to minimise costs. As the number of uses of the dummy blocks increases, there is the risk of chipping of coating material from the dummy blocks. This chipping or these particles lead to contamination of the actual diode laser blocks and thus to failures.e) The use of dummy blocks represents an additional source of error in the overall process for the machining.
The dummy blocks can prevent the difficulty of shading only on one of the cleavage surfaces to be coated, but not on both cleavage surfaces. Thus, the difficulty of shading remains unsolved.
US 2005/0221549 A1 discloses a method for the production of diode laser bars in which an anti-adhesive material or platinum is applied to corners or edges of the individual chips in order to prevent thermocompressive bonding of the stacked bars based on the direct contact of extensive gold surfaces on one another.
However, the anti-adhesive material or platinum applied to the corners or edges leads to the drawback that, again, a continuous coating layer is present on the facets which can disadvantageously influence the unstacking of the diode laser bars and also lead to tearing of the coating.
It is known from US 2005/0064090 A1 to prevent shading of the facets during the coating by arranging additional dummy blocks between the individual diode laser bars. Furthermore, it is known to etch trenches into the back of the diode laser bars. However, such etching of trenches leads to a reduction in yield if the wafer, which is just 0.1 mm thick, is further machined photolithographically and wet-chemically using the method described in US 2005/0064090 A1, so that overall no improvement of the total production process is obtained.